In the passage below, shouldn't the writer have included which is before the word championed, since it is in a non-defining clause?

In seeking to describe the origins of theater, one must rely primarily on speculation, since there is little concrete evidence on which to draw. The most widely accepted theory, championed by anthropologists in the late nineteenth and early twentieth centuries, envisions theater as emerging out of myth and ritual. The process perceived by these anthropologists may be summarized briefly.

1 Answer
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No, a non-finite clause as well as a finite clause can be a supplementary clause (which is what you call 'a non-defining clause'). And if you're to use a finite clause there, it should be not which is but which was, because the supplementary clause is about the late nineteenth and early twentieth centuries.

The most widely accepted theory, which was championed by anthropologists in the late nineteenth and early twentieth centuries, envisions theater as emerging out of myth and ritual.

EDIT

Here are some of the many, many examples found in news articles and books:

Tyndall believed it was vital for improving human health that the germ theory, championed by people like Louis Pasteur and Joseph Lister, be shown to be true.

The other dominant theory, championed by anthropologist Donald Symons in his 1979 book The Evolution of Human Sexuality, holds that the female orgasm, like male nipples, evolved as a byproduct of natural selection.

@Kris Saying that it "appears between a pair of commas and is parenthetical" is the same thing as saying that it's a supplementary clause. Then, why do you tell me to see your comment??
– JK2Feb 6 at 8:32

Because that's a much simpler and straightforward way to look at it.
– KrisFeb 6 at 8:43

@Kris How the mouthful, laymanish "appears between a pair of commas and is parenthetical" is much simpler and straightforward than the single, authentic term "supplementary"?
– JK2Feb 6 at 10:05

Thanks a lot, but some say that it's essential to include "which was" because it's in a non-defining clause, and it should not be omitted?? is that right?
– Adam JordanFeb 6 at 12:55